An active probe is a device for connecting a measurement tool, such as an oscilloscope, data analyzer, voltmeter, ammeter, etc., to a device under test (DUT), which may be an electrical component, printed circuit board (PCB), and the like. Active probes are designed to minimize the effect that the presence of the measuring tool has on the system being measured, and typically include sensors and buffers. For example, an active probe for measuring voltage may have a high impedance sensor coupled with an active circuit such as an operational amplifier, or op-amp, to amplify the sensed voltage to levels that may be read by the measurement tool. Reactive and resonant circuits, for example, may be monitored using an active probe in situations where the mere presence of a normal probe would change the resonant frequency of the reactive circuit because the normal probe itself has a non-trivial resistance, capacitance, inductance, and/or impedance. Thus, unlike a passive probe, which may be just a simple conductor, active probes tend to be large relative to the DUT or components of the DUT—so large and unwieldy that it becomes difficult to connect the active probe to smaller and smaller devices that are packed at higher and higher densities on a PCB.
There are disadvantages associated with conventional methods for attaching an active probe to a DUT, shown in FIGS. 1A through 1D.
FIG. 1A illustrates a conventional approach to attaching an active probe 100 to a signal trace of interest 102 on the surface of a printed circuit board PCB 104 of a device under test via an adapter 106 that must be soldered to PCB 104. This approach provides good performance but requires not only foreknowledge of what signal traces will need to be monitored but also a not-insignificant amount of space on PCB 104 for one or more adapters. As the density of elements on PCBs increases, this approach becomes prohibitive in terms of space and cost. This approach has the additional disadvantage that adapter 106 forces active probe 100 into an orientation that is normal to the surface of PCB 104, which could be a problem when probing multiple cards in a cage, for example, where the distance between boards may be too small to accommodate an active probe in that orientation.
FIG. 1B illustrates another conventional approach to attaching active probe 100 to PCB 104, i.e., by providing a series of test points 108 on the surface of PCB 104, which the user can probe by holding active probe 100 so that the probe pin 110 contacts the signal of interest while a ground blade 112 contacts a ground plane or reference signal. While this approach takes up less space than the approach shown in FIG. 1A, the approach shown in FIG. 1B has the disadvantage that the user must hold the probe in place manually or by using articulated clamping devices, for example. If multiple probes must be held in place, multiple users or claims must be present to hold the probes in place, which quickly becomes unwieldy.
FIGS. 1C and 1D illustrate other conventional approaches to attaching active probe 100 to PCB 104 by using a flexible signal lead 114 connected to the signal input pin of active probe 100. In FIG. 1C, lead 114 is fitted with a clamp, hook, or other device 116 for attaching the end of the lead to an available pin 118 of a device on PCB 104. The reference input (e.g., the ground pin) of active probe 100 is attached to a ground or reference pin 120 that is soldered onto PCB 104. In FIG. 1D, lead 114 includes a socket that fits over a test point 122 that is provided on PCB 104. In FIGS. 1C and 1D, while better than the approaches shown in FIGS. 1A and 1B, the conventional approaches shown in FIGS. 1C and 1D have the disadvantage that the flexible signal lead 114, which is not shielded, reduces the performance of active probe 100 due to injection of noise or reduction of bandwidth caused by the resistance, capacitance, or impedance of lead 114, which increase as the length of lead 114 increases. In addition, the distance between the far end of lead 114 and reference pin 120 can cause ringing of the measured signal or the creation of a ground loop.
Accordingly, there exists a need for an active probe adapter that overcomes the disadvantages associated with conventional approaches to attaching an active probe to a device under test.